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PT1.S3.Q11 - water vapor evaporated from the ocean contains

Alum Member
edited April 2016 276 karma
Can someone explain the reasoning behind answer choice (B)? I'm not 100% sure why oxygen-18 in seawater is increased, not decreased.

• Alum Inactive ⭐
1392 karma
Here’s how I visualized this question:

You have the Pacific Ocean. For the sake of understanding this question, let’s assume there’s an equal amount of O-16 and O-18 in the Pacific Ocean to start off.

According to the stimulus, when the ocean water evaporates, it has a higher portion of O-16 than O-18.

Let’s say 90% to 10% is the advantage of O-16 over O-18.

When this happens, the ocean water that is NOT evaporated has a higher portion of O-18 because for whatever reason it did not make it into the vapor state.

Ordinarily, this would not matter because the water would simply return to the Pacific Ocean through seawater that received precipitation (rain, snow, etc).

However, during the ice age, this precipitation would be trapped in the ice. Thus, the O-16 that would ordinarily precipitate back to the seawater and flow into the ocean does not. As a result, the ocean water has a higher level of O-18.

This is the answer in B.

This question worked well in my head, but I’m not sure if I explained it properly. If I did not, please say so!

@runiggyrun @Micaela_OVO @cantgetright
• Alum Member
edited April 2016 1018 karma
@"The 180 Bro_OVO" I had a similar approach.

So the composition of O16 and 18 in the ocean normally stays the same. A greater amount of O16 than O18 evaporates, but the composition doesn't change because that evaporated water comes back to the ocean by precipitation.

During an Ice Age *dun dun dun*, the evaporated water never makes it back to the ocean. What happened to the composition? Remember that a greater amount of O16 than O18 evaporates. Naturally, we see a greater concentration of O18 than before!
• Alum Inactive Sage Inactive ⭐
2481 karma
Both @Micaela_OVO and @"The 180 Bro_OVO" explained it perfectly.
Just in case that wasn't visual enough, imagine the ocean is purple from having red O16 and blue O18. Vapor is more red than blue (more O16, less O18). Generally it doesn't matter, because all the red falls back in the ocean as rain, so the ocean stays purple.
In an ice age, some of that red gets trapped in the ice, so you presumably end up with red ice (rich in O16) and the ocean becomes increasingly blue (rich in O18) as more and more of the red evaporates and doesn't come back.
• Alum Member
edited April 2016 276 karma
Thank you for responding guys! That color analogy was truly creative and awesome! I think this is one of those questions that test your understanding of proportion. Here's how I summarized this question:

During an Ice Age

*Heavier oxygen 18: Less readily able to evaporate
= less trapped in ice
= more in seawater

*Lighter oxygen 16: More readily able to evaporate
= more trapped in ice
= less in seawater

So when the Ice Age ends, the overall composition of the ocean would change again to more oxygen 16 in seawater than oxygen 18.
• Yearly Member
4 karma

Hi guys! Thank you all very much for the explanations - however I still seem to be having issues at the very end of the of the explanations...

So the correct answer choice (B) is "The concentration of O-18 in the seawater is increased".

And I understand everything in The 180 Bro_OVO's explanation right up until the part where it says "This is the answer in B.", because the conclusion right before this part is "As a result, the ocean water has a higher level of O-18."

I'm so sorry if I'm the only one who cant seem to grasp the logic behind this jump, but shouldn't answer choice B say "OCEAN" instead of "seawater"? What am I missing?

Thank you so so much!

• Alum Member
298 karma

@AlaraE_95 said:
Hi guys! Thank you all very much for the explanations - however I still seem to be having issues at the very end of the of the explanations...

So the correct answer choice (B) is "The concentration of O-18 in the seawater is increased".

And I understand everything in The 180 Bro_OVO's explanation right up until the part where it says "This is the answer in B.", because the conclusion right before this part is "As a result, the ocean water has a higher level of O-18."

I'm so sorry if I'm the only one who cant seem to grasp the logic behind this jump, but shouldn't answer choice B say "OCEAN" instead of "seawater"? What am I missing?

Thank you so so much!

IMO ocean water and seawater are one & the same. Sometimes you'll find the LSAT might change verbiage in this way & so you have to be a little open-minded when it comes to the AC's. It might not state verbatim the words in the stimulus; you have to make the assumption that they are synonymous, which I don't think is an unreasonable jump to make in this scenario. Hope that helps!

• Yearly Member
4 karma

@"Jay Tee" said:

@AlaraE_95 said:
Hi guys! Thank you all very much for the explanations - however I still seem to be having issues at the very end of the of the explanations...

So the correct answer choice (B) is "The concentration of O-18 in the seawater is increased".

And I understand everything in The 180 Bro_OVO's explanation right up until the part where it says "This is the answer in B.", because the conclusion right before this part is "As a result, the ocean water has a higher level of O-18."

I'm so sorry if I'm the only one who cant seem to grasp the logic behind this jump, but shouldn't answer choice B say "OCEAN" instead of "seawater"? What am I missing?

Thank you so so much!

IMO ocean water and seawater are one & the same. Sometimes you'll find the LSAT might change verbiage in this way & so you have to be a little open-minded when it comes to the AC's. It might not state verbatim the words in the stimulus; you have to make the assumption that they are synonymous, which I don't think is an unreasonable jump to make in this scenario. Hope that helps!

Yes this helps, thank you so much!!

• Alum Member
227 karma

I struggled with this question for the same reason as @AlaraE_95. I was thrown off as I thought the ocean and the sea are two distinct things as I read through the question. I'm glad I read all the responses here.

• Alum Member
6 karma

UGH THEY TOTALLY GOT ME! I thought we were comparing ocean vapor versus sea vapor. dang.

• Alum Member
4 karma

Thank you for the explanations, I was tricked by the usage of seawater instead of ocean water and thought it was two separate things. It is not.